jemi

jemi: a compact, pure-C JSON serializer for embedded systems

Overview

jemi makes it easy to construct complex JSON structures and then emit them to a buffer, a string or a stream. Designed specifically for embedded systems, jemi is:

• compact: one source file and one header file
• portable: written in pure C
• deterministic: jemi uses user-provided data structures and never calls malloc().
• yours to use: jemi is covered under the permissive MIT License.

jemi derives much of its efficiency and small footprint by a philosophy of trust: Rather than provide rigorous error checking of input parameters, jemi instead assumes that you provide valid parameters to function calls.

A Short Example

jemi provides two styles for building JSON structures. The "all in one" style shown in jemi_example1() lets you use C syntax that parallels the resulting JSON structure. The "piecewise" style shown in jemi_example2() shows how you can pre-build pieces of JSON structure to be emitted later. Both examples emit the same results.

#include "jemi.h"
#include <stdio.h>

#define JEMI_NODES_MAX 30
static jemi_node_t node_pool[JEMI_NODES_MAX];

/**
 * Using the "all in one" constructors to create a compound JSON structure.
 */
void jemi_example1(void) {
    jemi_init(node_pool, JEMI_NODES_MAX);

    jemi_node_t *root =
        jemi_object(
            jemi_string("colors"),
            jemi_object(
                jemi_string("yellow"),
                jemi_array(jemi_integer(255), jemi_integer(255), jemi_integer(0), NULL),
                jemi_string("cyan"),
                jemi_array(jemi_integer(0), jemi_integer(255), jemi_integer(255), NULL),
                jemi_string("magenta"),
                jemi_array(jemi_integer(255), jemi_integer(0), jemi_integer(255), NULL),
                NULL),
            NULL);
    jemi_emit(root, (void (*)(char))putchar); // casting avoids compiler warning
    putchar('\n');
}

/**
 * Using the "append" functions to piecewise build a compound JSON structure.
 */
void jemi_example2(void) {
    jemi_node_t *root, *dict, *rgb;

    jemi_init(node_pool, JEMI_NODES_MAX);

    // build from the inside out...
    root = jemi_object(NULL);
    jemi_object_append(root, jemi_list(jemi_string("colors"), dict = jemi_object(NULL), NULL));
    jemi_object_append(dict, jemi_list(jemi_string("yellow"), rgb = jemi_array(NULL), NULL));
    jemi_array_append(rgb, jemi_list(jemi_integer(255), jemi_integer(255), jemi_integer(0), NULL));
    jemi_object_append(dict, jemi_list(jemi_string("cyan"), rgb = jemi_array(NULL), NULL));
    jemi_array_append(rgb, jemi_list(jemi_integer(0), jemi_integer(255), jemi_integer(255), NULL));
    jemi_object_append(dict, jemi_list(jemi_string("magenta"), rgb = jemi_array(NULL), NULL));
    jemi_array_append(rgb, jemi_list(jemi_integer(255), jemi_integer(0), jemi_integer(255), NULL));

    jemi_emit(root, (void (*)(char))putchar); // casting avoids compiler warning
    putchar('\n');
}

int main(void) {
    jemi_example1();
    jemi_example2();
}

Both example1() and example2() output the same JSON, which (when formatted for redability) looks like this:

{
   "colors":{
      "yellow":[
         255,
         255,
         0
      ],
      "cyan":[
         0,
         255,
         255
      ],
      "magenta":[
         255,
         0,
         255
      ]
   }
}

Advanced Example

Suppose you know the overall JSON structure that you're going to use, but you want to dynamically add data to various nodes within the structure before emitting it.

jemi handles this case for you with the following features;

• jemi_copy() makes a copy of any jemi structure (or substructure)
• jemi_list() lets you create a "disembodied list" of items that can be appended to the body of a jemi_list or jemi_object via jemi_array_append() and jemi_object_append().
• jemi_xxx_set() lets you update the value of a string, boolean or number node.
• The C syntax a = fn() evaluates fn() and assigns it to a and the entire expression evaluates to that value. As shown in the example below, you can exploit this to save references to jemi sub-expressions in the JSON structure.

The result isn't especially pretty, but in practice, you'd create bespoke functions to fill in and copy your templates and to insert them into your overall structure.

jemi_node_t *snippet_template, *snippets, *color_map, *color_name, *colors,
            *red_val, *grn_val, *blu_val;

// create a snippet that will be used as a key/value pair in an object.
snippet_template =
    jemi_list(
        color_name = jemi_string("color_name"),
        jemi_array(red_val=jemi_integer(0),
                   grn_val=jemi_integer(0),
                   blu_val=jemi_integer(0),
                   NULL),
        NULL);

// Define the overall JSON structure for our color map
color_map =
    jemi_object(
        colors = jemi_string("colors"),
        snippets = jemi_object(NULL),
        NULL);
ASSERT(renders_as(color_map, "{\"colors\":{}}"));

// customize template for yellow and add a copy to the "snippets" sub-structure
jemi_string_set(color_name, "yellow");
jemi_integer_set(red_val, 255);
jemi_integer_set(grn_val, 255);
jemi_integer_set(blu_val, 0);
jemi_object_append(snippets, jemi_copy(snippet_template));
ASSERT(renders_as(color_map, "{\"colors\":{"
                                  "\"yellow\":[255,255,0]"
                                  "}}"));

// customize template for cyan and add a copy to the "snippets" sub-structure
jemi_string_set(color_name, "cyan");
jemi_integer_set(red_val, 0);
jemi_integer_set(grn_val, 255);
jemi_integer_set(blu_val, 255);
jemi_object_append(snippets, jemi_copy(snippet_template));
ASSERT(renders_as(color_map, "{\"colors\":{"
                             "\"yellow\":[255,255,0],"
                             "\"cyan\":[0,255,255]"
                             "}}"));

// customize template for magenta and add a copy to the "snippets" sub-structure
jemi_string_set(color_name, "magenta");
jemi_integer_set(red_val, 255);
jemi_integer_set(grn_val, 0);
jemi_integer_set(blu_val, 255);
jemi_object_append(snippets, jemi_copy(snippet_template));
ASSERT(renders_as(color_map, "{\"colors\":{"
                             "\"yellow\":[255,255,0],"
                             "\"cyan\":[0,255,255],"
                             "\"magenta\":[255,0,255]"
                             "}}"));

No Guard Rails

jemi trusts that you know what you're doing and that you'll pass valid arguments to the functions. As such, jemi does very little error checking. If your app warrants additional error checking, by all means, write wrappers around the jemi functions to perform whatever level of safety you need.

With that said, there are a few things to keep in mind:

• Don't forget to pass NULL as the last argument to jemi_array(), jemi_object(), or jemi_list(). Failure to do so will almost certainly result in some sort of segfault.
• Calling jemi_float_set() on a node that's not a number will lead to unexpected results. Ditto for integer, string and boolean.
• Calling jemi_array_append() on a node that's not an array will lead to unexpected results. Ditto for jemi_object_append(). However, it is okay to call jemi_list_append() on any jemi node type or NULL.
• The JSON spec requires that each key of a key/value pair in an object is a string. However, jemi_object() does not enforce this: you can use any jemi object as a key. Furthermore, it does not check to see that every key has a corresponding value.